Rapper 50 Cent says he learned everything he needed to know to make it in the music business selling crack on the streets of Queens. In “From Pieces to Weight: Once Upon a Time in Southside Queens,” his drugs-to-riches memoir, which hits bookstores next week, Fitty describes his journey from pusher to pop star – and all the run-ins with police and rivalries in between. JEREMY OLSHAN reports.

CURTIS Jackson III never knew his father.

His mother terrified him. At age 5, when he was beat up by another kindergartner and came home crying, she told him next time “you pick something up and hit him if you have to. But you’re not going to come back here crying.”

Three years later, his mother, a drug dealer herself, was murdered by carbon-monoxide poisoning. Rival thugs drugged her in her own home and turned on the gas.

The hyperactive orphan, who went by the name Boo-Boo, was raised by his grandparents, who lived on 161st Street in South Jamaica.

It was outside this house that Jackson was shot nine times, propelling him into the spotlight where he now resides as the multiplatinum rapper 50 Cent.

His life now is a far cry from the time his aunt poisoned his dog, Dillinger, because the Doberman ate the entire Thanksgiving turkey before dinner was served.

“It happened so much, it seemed normal. I thought that everybody’s family sat around, got drunk, and played practical jokes that caused second-degree burns,” he writes.

But even in his junior-high-school days, 50 knew he wanted to “get rich,” or – as his debut album put it – “die trying.”

Looking around the neighborhood, he quickly came to the conclusion that the best and only way to get ahead was the drug game.

He says he learned more in one day on the street than a year in the classroom.

“Nothing I was being told in school made sense to my reality,” he writes. “I could break down a kilogram of cocaine into ounces, grams or any combination of the two. That’s how I learned my fractions and metric conversion, through real-life applications.”

By 10th grade he dropped out of Andrew Jackson HS in Cambria Heights, but not before getting busted for having several vials of crack hidden in his sneaker.

Coming of age just in time for crack to hit the East Coast, 50 quickly moved up the ladder, learning the business from the veterans before, one by one, they were locked up or killed.

“There’s a rhythm to making crack. And with each step, my heart beat faster because all I really wanted to do was sell it. I’d whip up the mix, boil the water, cook it up, piece out the rocks, stuff the vials and hit the block,” he writes.

The aspiring drug mogul watched how competitors pushed their rivals out of the neighborhood with a combination of muscle and marketing.

HE graduated from spending his earnings on Air Jordans to Mercedes, and relished the rush of eluding police in the occasional high-speed chase.

“At the slightest infraction, I was punching someone in the face . . . The calmer and more confident I became, the more I fought,” he writes.

Then he had to tell his grandmother he wanted to stop attending church.

“I didn’t know how to explain to her that it didn’t make sense for me to praise the Lord in the pews only to turn around and sell crack on the highways and in the hedges.”

Inevitably, he was busted. But he managed to avoid hard time by getting sent to a two-year program for drug offenders.

When he got out, 50 planned to return to selling and supplying crack, hustling for a buck on the stretch of Guy Brewer Boulevard in South Jamaica known as “The Strip.”

But then, in a chance meeting with Jam Master Jay, the hip-hop pioneer of Run-DMC fame, 50 instantly decided he was going to be a rapper.

He offers little explanation for this shift in career trajectory. He mentions no prior interest in music. He just decided.

But first he needed a name. So he stole one from a dead gangster.

“The real 50 Cent was a stickup kid from Brooklyn who used to rob rappers,” he explained.

“Other rappers were running around, calling themselves Al Capone and John Gotti and Pablo Escobar. If I was going to take a gangster’s name, then I wanted it at least to be that of someone who would say, ‘What’s up?’ to me on the street if we ever crossed paths.”

Under Jay’s tutelage he learned the basics, but soon realized that if he wanted something to happen, he was going to have to do it himself.

But when Jay refused to let 50 out of his contract unless he paid $50,000, he realized business is sometimes rougher than the street.

“Sometimes the rap game does remind me of the crack game,” he said.

“But at least in the crack game, you can lay on somebody when he plays you out of pocket. The music industry had a whole sepa- rate set of rules that I had to adjust to.”

So 50 decided to make a name for himself the only way he knew how.

“The only business model I had was from selling drugs, so that’s how I marketed my product,” he writes.

“I knew the only way to get into any market is to give out free samples. I had to build up a clientele before I could see a profit. I had to invest in my brand.”

His music became so popular on free tapes he sent to DJs – long before he had ever sold a record – that he considered returning to the drug business so he would have enough money to keep up appearances.

“How would I look if I had the hottest single in the city but was still riding dollar vans?” he writes.

At the same time, he began the most vicious of his rapper rivalries with Ja Rule. The two have long differed on how it started, but the way 50 tells it, a friend of his robbed Ja Rule.

THE battle escalated in a series of rap songs dissing each other, but was overshadowed by 50’s date with nine Bullets.

He got into a car outside his grandparent’s home on May 24, 2000, and the shooter came up and began firing.

“I felt my legs on fire,” 50 recalls.

His attacker was murdered several weeks later.

Fitty admits the shooting gave him “cred” among music fans the same way it did on the street, but that doesn’t mean it was his defining moment. And if he had 50 cents for every time someone asked him “how it felt to be shot,” the question would not irritate him nearly as much as it does.

“Honestly, it didn’t feel good,” he writes. “Now it’s just a memory, but when it happened, it hurt. Bad.”

After months of rehabilitation and working out in the gym, 50 returned to the music scene more determined than ever, with lyrics critics said were more harsh and true to life on the street than anything else at the time.

“Truth is, there is no such thing as a ‘gangsta rapper,’ ” 50 writes. “Because no one can be a gangsta and a rapper at the same time. A rapper can have gangsta ties, he can know gangsta, but he can’t be a gangsta. I was trying to figure out which I was going to be. It’s like I had a microphone in one hand and my Ruger in the other.”

Eminem, whom 50 credits with much of his success, heard some of these tracks, and immediately signed 50 to his new label.

HIS first album, “Get Rich or Die Trying,” sold more than 12 million copies, and his subsequent CDs fared almost as well.

Unable to escape controversy – by choice or not – 50 regularly wears a bulletproof vest, and drives in bulletproof vehicles.

When Jam Master Jay was murdered, 50 was questioned, he says, because of rumors the killer was trying to send him a message.

He’s had run-ins with the NYPD, most notably when cops found guns in his car outside a Manhattan nightclub.

Despite all this, the 30-year-old is currently filming a movie loosely based on his life, directed by Oscar winner Jim Sheridan. Toronto may have to stand in for Queens, however.

“I hate going to perform in New York City now,” 50 said. “Like I said, cops follow me everywhere I go. But even though I don’t like going to NYC announced, I never forget exactly where I came from and who helped me get where I am now.”

Fitty’s violent past has been turned into a marketing mythology, but the truth of it is what sets his music apart from the fakers, he explains.

“With me, 90 percent of my music is real, and only about 10 percent of it I’ll embellish,” he said.

He doesn’t say if the same ratio applies to his memoir.

“From Pieces to Weight” goes on sale Tuesday. It was written with Kris Ex.